1. Field of the Invention
The present invention relates to a visualization technique for co-rendering multiple attributes in real time, thus forming a combined image of the attributes. The combined image is visually intuitive in that it distinguishes certain features of an object that are substantially indistinguishable in their natural environment.
2. Related Art
In the applied sciences, various fields of study require the analysis of two-dimensional (2-D) or three-dimensional (3-D) volume data sets wherein each data set may have multiple attributes representing different physical properties. An attribute, sometimes referred to as a data value, represents a particular physical property of an object within a defined 2-D or 3-D space. A data value may, for instance, be an 8-byte data word which includes 256 possible values. The location of an attribute is represented by (x, y, data value) or (x, y, z, data value). If the attribute represents pressure at a particular location, then the attribute location may be expressed as (x, y, z, pressure).
In the medical field, a computerized axial topography (CAT) scanner or magnetic resonance imaging (MRI) device is used to produce a picture or diagnostic image of some specific area of a person's body, typically representing the coordinate and a determined attribute. Normally, each attribute within a predetermined location must be imaged separate and apart from another attribute. For example, one attribute representing temperature at a predetermined location is typically imaged separate from another attribute representing pressure at the same location. Thus, the diagnosis of a particular condition based upon these attributes is limited by the ability to display a single attribute at a predetermined location.
In the field of earth sciences, seismic sounding is used for exploring the subterranean geology of an earth formation. An underground explosion excites seismic waves, similar to low-frequency sound waves that travel below the surface of the earth and are detected by seismographs. The seismographs record the time or arrival of seismic waves, both direct and reflected waves. Knowing the time and place of the explosion, the time of travel of the waves through the interior can be calculated and used to measure the velocity of the waves in the interior. A similar technique can be used for offshore oil and gas exploration. In offshore exploration, a ship tows a sound source and underwater hydrophones. Low frequency (e.g., 50 Hz) sound waves are generated by, for example, a pneumatic device that works like a balloon burst. The sounds bounce off rock layers below the sea floor and are picked up by the hydrophones. In either application, subsurface sedimentary structures that trap oil, such as faults and domes are mapped by the reflective waves.
The data is collected and processed to produce 3-D volume data sets. A 3-D volume set is made up of “voxels” or volume elements having x, y, z coordinates. Each voxel represents a numeric data value (attribute) associated with some measured or calculated physical property at a particular location. Examples of geological data values include amplitude, phase, frequency, and semblance. Different data values are stored in different 3-D volume data sets, wherein each 3-D volume data set represents a different data value. In order to analyze certain geological structures referred to as “events” information from different 3-D volume data sets must be separately imaged in order to analyze the event.
Certain techniques have been developed in this field for imaging multiple 3-D volume data sets in a single display, however, not without considerable limitations. One example includes the technique published in The Leading Edge called “Constructing Faults from Seed Picks by Voxel Tracking” by Jack Lees. This technique combines two 3-D volume data sets in a single display, thereby restricting each original 256-value attribute to 128 values of the full 256-value range. The resolution of the display is, therefore, significantly reduced, thereby limiting the ability to distinguish certain events or features from the rest of the data. Another conventional method combines the display of two 3-D volume data sets, containing two different attributes, by making some data values more transparent than others. This technique becomes untenable when more than two attributes are combined.
Another technique used to combine two different 3-D volume data sets in the same image is illustrated in U.S. Pat. No. 6,690,820 (“'820 patent”) assigned to Landmark Graphics Corporation and incorporated herein by reference. The '820 patent describes a technique for combining a first 3-D volume data set representing a first attribute and a second 3-D volume data set representing a second attribute in a single enhanced 3-D volume data set by comparing each of the first and second attribute data values with a preselected data value range or criteria. For each data value where the criteria are met, a first selected data value is inserted at a position corresponding with the respective data value in the enhanced 3-D volume data set. For each data value where the criteria are not met, a second selected data value is inserted at a position corresponding with the respective data value in the enhanced 3-D volume data set. The first selected data value may be related to the first attribute and the second selected data value may be related to the second attribute. The resulting image is an enhanced 3-D volume data set comprising a combination or hybrid of the original first 3-D volume data set and the second 3-D volume data set. As a result, the extra processing step needed to generate the enhanced 3-D volume data set causes interpretation delays and performance slow downs. Furthermore, this pre-processing technique is compromised by a “lossy” effect which compromises data from one seismic attribute in order to image another seismic attribute. Consequently, there is a significant loss of data visualization.
In non-scientific applications, techniques have been developed to define surface details (texture) on inanimate objects through lighting and/or shading techniques. For example, in the video or computer graphics field, one technique commonly used is texture mapping. Texture typically refers to bumps, wrinkles, grooves or other irregularities on surfaces. Textured surfaces are recognized by the way light interacts with the surface irregularities. In effect, these irregularities are part of the complete geometric form of the object although they are relatively small compared to the size and form of the object. Conventional texture mapping techniques have been known to lack the necessary surface detail to accomplish what is conventionally meant by texture. In other words, conventional texture mapping techniques provide objects with a colorful yet flat appearance. To this end, texture mapping was expanded to overcome this problem with what is now commonly referred to as a bump mapping.
Bump mapping is explained in an article written by Mark Kilgard called “A Practical and Robust Bump Mapping Technique for Today's GPU's” (hereinafter Kilgard) which is incorporated herein by reference. In this article, bump mapping is described as “a texture-based rendering approach for simulating lighting effects caused by pattern irregularities on otherwise smooth surfaces.” Kilgard, p. 1. According to Kilgard, “bump mapping simulates a surface's irregular lighting appearance without the complexity and expense of modeling the patterns as true geometric perturbations to the surface.” Kilgard, p. 1. Nevertheless, the computations required for original bump mapping techniques proposed by James Blinn in 1978 are considerably more expensive than those required for conventional hardware texture mapping. Kilgard at p. 2.
In view of the many attempts that have been made over the last two decades to reformulate bump mapping into a form suitable for hardware implementation, Kilgard proposes a new bump mapping technique. In short, Kilgard divides bump mapping into two steps. First, a perturbed surface normal is computed. Then, a lighting computation is performed using the perturbed surface normal. These two steps must be performed at each and every visible fragment of a bump-mapped surface. Kilgard.
Although Kilgard's new technique may be suitable for simulating surface irregularities (texture) representative of true geometric perturbations, it does not address the use of similar lighting effect to distinguish certain features of an object that are substantially indistinguishable in their natural environment.